The present invention relates generally to shields for magnetoresistive sensors used in electronic data storage and retrieval systems. In particular, this invention relates to biased domain wall free shields. The present invention is a further improvement on U.S. Pat. No. 6,437,949, herein incorporated by reference.
In an electronic data storage and retrieval system, a transducing head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically encoded information stored on a magnetic disc. MR sensors are comprised of at least one magnetic layer whose magnetic alignment responds to external applied magnetic fields. Typical MR sensors are of the anisotropic magnetoresistive (AMR) sensor or giant magnetoresistive (GMR) sensor type. When an MR sensor is passed over the surface of a magnetic storage disc, magnetic flux from the surface of the disc alters the magnetic alignment of the MR sensor, which in turn alters the magnetic resistivity of the MR sensor. The change in resistivity of the MR sensor can be detected by passing a current through the MR sensor and measuring a voltage across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary.
A response curve of the MR sensor compares the voltage across the MR sensor to the magnetic flux received from the disc by the sensor. This response curve has both linear and non-linear portions, of which it is preferred that the MR sensor operate along the linear portion. To force the MR sensor to operate along the linear portions, the sensor is magnetically biased at a biasing point that is located along the linear portion of the response curve.
MR sensors have a large read pulse width that must be reduced for high linear densities. The pulse width of the MR sensor is reduced by sandwiching the MR sensor between two magnetic shields. The magnetic shields generally incorporate soft magnetic materials for their permeability to magnetic flux. The purpose of the shields is to prevent the magnetic flux from sources other than the desired transition from interacting with the MR sensor. During a read operation, the upper and lower shields ensure that the MR sensor reads only the information (transition) stored directly beneath it on a specific track of the magnetic disc medium by absorbing any stray magnetic fields emanating from adjacent tracks and transitions.
Within a conventional shield exists a plurality of magnetic domains separated from each other by a plurality of magnetic domain walls. Each domain has a magnetization that is oriented in a direction different than the magnetization of all adjacent domains. As the domain walls move, the bias point, as well as the response of the MR sensor to signals emanating from the magnetic disc medium changes. The overall result is noise during the read operation.
To avoid the problems associated with domain wall movement, the ideal shield structure would have no domain walls. Attempts to achieve a single domain structure in MR sensor shields have been largely unsuccessful due to inability of weak exchange coupling interactions to overcome large demagnetizing fields in conventional structures. The result is multiple domains, noise and suppression of sensor signal. Consequently, there remains a need in the art for a domain wall free shield without suppressing MR sensor amplitude while reducing or eliminating sources of noise.